Biochemical studies on the viral structure and the mechanism of replication of avian oncoviruses are proposed. The following four areas will be emphasized: (1) The gene sequences of the viral RNA genome will be studied by heteroduplex mapping, oligonucleotide fingerprinting of the viral RNA, and the peptide mapping of the viral glycoproteins. The sequence relationship among different strains of viruses, particularly with regard to the env gene will be emphasized. (2) The mechanism of viral RNA-dependent DNA synthesis in vitro will be studied by electron microscopy and biochemical characterization of the transcription complex and the DNA products. Particular emphasis will be addressed to the 5' yields 3' jumping mechanism, and the mechanism of synthesis and the structure of the full-genome-length complementary DNA. (3) Isolation and characterization of the transformation defective (td) mutants with different degrees of src depletion will be attempted. These td mutants will be studied with regard to the origin of the residual src sequences and the insertion of the cellular sequences. This study will be expected to help elucidate the mechanism of the deletion of the src gene. (4) The biological function of the in vitro synthesized complementary DNA will be studied by transfection, marker rescue, in vitro translation. This will reveal the structure and the genetic arrangement of the biologically active form of the viral DNA.